1. Technical Field
This invention relates to sensors for indicating concentrations of liquids, vapors, gases and the like, and more particularly to a thermo-electrically regulated adsorptive sensing method and an adsorptive sensing device usable therewith.
Accurate and rapid detection of concentrations of liquids, vapors, gases and the like is necessary in a great many industrial, medical, safety and other applications. For instance, safe yet effective medical anesthesia requires accurate, continuing and rapidly responsive indication of the concentration of anesthetic in the gas mixture inhaled by the patient. Relatively small increases or decreases in concentration of the anesthetic often require immediate corrective response from attending medical personnel. If such response is delayed, such as by failure to detect the change in anesthetic concentration, the success of the medical procedure can be endangered and the patient's life may be placed at risk. Optimal performance of a medical anesthetic sensor is characterized by high sensitivity to and accurate indication of relatively small increases and decreases in concentration of the anesthetic. Many other applications for sensing devices require comparable sensitivity, accuracy and response time.
2. Description of the Prior Art
Dolan U.S. Pat. No. 3,045,198, issued July 17, 1962, discloses an adsorptive sensing element having a stratum of exposed, electrically conductive, discrete, adsorbent particles anchored on the non-conductive nonabsorbent resilient surface of a base. The particles are anchored such that under normal conditions adjacent particles are in conductive contact whereby at least one but usually many electrical paths extend across the stratum between spaced apart electrical leads, each such lead contacting a broad portion of the stratum. Exposure of the adsorptive element to a selected substance to be detected causes a portion of such substance to be adsorbed onto the surfaces of the particles, increasing the resistance of or reducing the current flow through the path or paths between the electrical leads of the adsorptive element. Dolan U.S. Pat. No. 4,129,030 discloses sensing apparatus including an adsorptive sensing element placed in a balanced bridge circuit for detection of small changes in the resistance of or the current flow through the adsorptive sensing element. Dolan U.S. Pat. No. 4,224,595 discloses an adsorptive sensing element having various sized abutting adsorbent particles for increasing the sensitivity of the adsorptive element. Dolan U.S. Pat. No. 4,237,721 discloses sensing apparatus including an adsorptive sensing element operated in its non-linear region for sensing substances having a Van der Waals' "a" constant of 9 or less, and a balanced bridge circuit for detecting changes in the resistance of or the current flow through the adsorptive element.
Adsorptive sensing elements display an inherent hysteresis effect, responding more rapidly to increases in the concentration of the substance of interest than to decreases therein. This hysteresis effect results because the adsorptive particles desorb portions of the substance more slowly than they adsorb the substance. Adsorptive sensing elements are also temperature sensitive, having greater sensitivity at lower temperatures.
Le Vine U.S. Pat. No. 3,096,473 discloses a method for operating a gas sensor which exhibits substantial response to contact adsorption by a given class of gases, in which the sensor is alternately operated at a lower sensing temperature to sense said gases and at a higher purging temperature to purge the sensor of such gases. Le Vine relies on passive cooling, presumably through radiation, convection and conduction, to return the adsorptive element from the higher purging temperature to the lower sensing temperature. The time required for the adsorptive element to return to the lower temperature determines the sampling rate of the sensor, and thus the minimum achievable response time of the sensor to changes in concentration of the gas. The Le Vine sensor is thus not suited to applications where changes in concentration can occur faster than the adsorptive element can be first heated and then passively cooled.